This invention relates to logarithmic rf amplifiers and, more particularly, to a logarithmic rf amplifier which employs differential amplifiers to provide full-wave detection of the rf signal.
Logarithmic rf amplifiers are frequently used in radar systems for compression of the dynamic range of received signals. The amplitude of a video output signal varies logarithmically with respect to the amplitude of the rf input signal. Logarithmic rf amplifiers usually comprise multiple rf amplification stages with amplitude limiting. The rf signals in each stage are detected and summed to provide a log video output.
A prior art logarithmic rf amplifier is disclosed in U.S. Pat. No. 3,668,535, Landsdowne, issued June 6, 1972. The rf outputs of each amplification stage are detected and applied to video limiters. The outputs of the video limiters are summed in a delay line arrangement. In other prior art log amplifiers, a single-ended amplifier transistor performs half-wave self-detection and the detected voltages are summed. Prior art log rf amplifiers have utilized half-wave detection with a peak holding capacitor.
Prior art log rf amplifiers have provided generally satisfactory performance but have been subject to certain disadvantages. The use of half-wave detection and peak holding capacitors makes the pulse response slow. Filter circuits to remove rf ripple also affect pulse response. In addition, the use of single-ended amplifier stages has often resulted in output transients and unsymmetrical limiting.
In general, the accuracy of a log rf amplifier, or the deviation of the transfer characteristic from an ideal log characteristic, should be less than 1%. In addition, log amplifier circuits are usually required to exhibit very fast pulse response and wide bandwidths. Furthermore, it is desirable for the logarithmic rf amplifier to exhibit symmetrical limiting characteristics while avoiding transients.
It is a general object of the present invention to provide a novel logarithmic rf amplifier.
It is another object of the present invention to provide a logarithmic rf amplifier utilizing differential amplifier stages and full-wave detection.
It is yet another object of the present invention to provide a logarithmic rf amplifier exhibiting extremely fast pulse response.